Method for reducing surface charge on semiconductor wafers to prevent arcing during plasma deposition

ABSTRACT

A process of forming a layer of conductive material over a layer of insulating material is provided. A wafer is positioned on a wafer platform such that it is thermally and electrically coupled to the wafer platform. A clamping ring engages the peripheral edge of the wafer such that the wafer is held against the top surface of the wafer platform. The clamping ring is electrically coupled to the wafer pedestal. The wafer is exposed to a plasma comprising conductive material and an initial layer of conductive material is formed over the insulating layer until the top surface of the wafer is electrically coupled to the clamping ring. The wafer pedestal is then electrically biased and additional conductive material is formed. Once the initial layer of conductive material is electrically coupled to the clamping ring, the potential difference between the top and bottom surface of the wafer is zero such that arcing through the wafer is reduced. The wafer platform may also be exposed to the plasma so as to reduce the potential difference between the top and bottom surfaces of the wafer when the wafer platform is electrically biased.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.08/929,476, filed Sep. 15, 1997, now U.S. Pat. No. 6,057,235, issued May2, 2000.

BACKGROUND OF THE INVENTION

The present invention relates in general to the fabrication ofsemiconductor wafers, and, more particularly, to a method of reducingstatic electric charges on the surface of wafers resulting from plasmadeposition.

Semiconductor processing requires the deposition of conductive materialson semiconductor wafers. Typically, plasma deposition, in the form ofphysical vapor deposition (PVD) or plasma enhanced chemical vapordeposition (PECVD), is used to deposit the conductive material on thesemiconductor wafer as processing temperatures are significantly lowerthan non-plasma deposition methods. Plasma deposition is carried out ina plasma chamber with the wafer being secured on top of a waferplatform. Typically, the wafer is physically secured to the waferplatform by a clamping ring which engages an outer portion of the topsurface of the wafer. The wafer platform is typically conductive so thatthe bottom surface of the wafer is electrically coupled to the waferplatform. The clamping ring is electrically coupled to the waferplatform such that the top surface of the wafer is also electricallycoupled to the wafer platform.

An electrical bias is then applied to the wafer platform in order tocontrol the plasma deposition process. The electrical bias sets up anegative potential on the wafer to attract the positive ions from theplasma. The topology of the formed layer is controlled by adjusting theapplied electrical bias. Typically, the conductive material is appliedover a layer of insulating material in order to form desired contacts,interconnects, or the like. While the clamping ring is electricallycoupled to the wafer platform, the top surface of the wafer is notshorted to the wafer platform as the top surface of the wafer isinsulated by the layer of insulating material. Once the plasma isgenerated, the top surface of the wafer is continually charged as theconductive material is deposited thereby forming a potential differencebetween the bottom surface of the wafer and the top surface of thewafer. This potential difference may lead to arcing through the wafercausing damage to one or more dies on the wafer.

Accordingly, there is a need for a process of depositing conductivematerials over an insulating layer in which the risk of arcing throughthe wafer is reduced. Preferably, such a process would be inexpensive,easy to implement, would not entail excess processing steps, and wouldnot adversely affect the quality of the deposited conductive layer.

SUMMARY OF THE INVENTION

The present invention meets this need by providing a method in which aninitial layer of conductive material is deposited over the insulatinglayer prior to electrically biasing the wafer platform. The initiallayer is preferably continuous so that the top surface of the wafercontacts the clamping ring, thereby, shorting the top surface to thewafer platform. Once the initial layer of conductive material is formed,the wafer platform is electrically biased to control the growth of therest of the conductive layer. The present invention also meets this needby providing a method in which the wafer platform is exposed to theplasma so as to reduce the potential difference between the top andbottom surfaces of the wafer.

According to a first aspect of the present invention, a process ofapplying conductive material over a layer of insulating material on asemiconductor wafer in which a bottom surface of the wafer iselectrically coupled to a wafer platform comprises forming a first layerof the conductive material over the insulating layer, the first layer ofconductive material electrically coupling a top surface of the wafer tothe wafer platform. The wafer platform is then electrically biased to apredetermined potential. A second layer of the conductive material isthen formed over the first layer of conductive material. The conductivematerial may comprise a metallic material, such as a refractory metal,or semiconductor material. The first layer may have a thickness of atleast 50 Angstroms and preferably, a thickness ranging from about 100Angstroms to about 200 Angstroms. The insulating layer may beelectrically coupled to the wafer platform by a clamping ring coupled toa portion of a top surface of the insulating layer and the waferplatform. The clamping ring is preferably electrically and physicallycoupled to the wafer platform. A portion of a top surface of the waferplatform may be substantially covered by the wafer and the clampingring.

According to another aspect of the present invention, a process ofapplying conductive material over a layer of insulating material on asemiconductor wafer in which a bottom surface of the wafer iselectrically coupled to a wafer platform comprises exposing the wafer toa plasma including a conductive material. A substantially continuousfirst layer of the conductive material is formed over the insulatinglayer, the first layer of conductive. material electrically coupling atop surface of the wafer to the wafer platform. The wafer platform iselectrically biased to a predetermined potential, and a second layer ofthe conductive material is then formed over the first layer ofconductive material. The step of exposing the wafer to a plasmaincluding a conductive material may comprise the step of exposing aportion of the wafer platform to the plasma.

According to yet another aspect of the present invention, a process ofapplying conductive material over a layer of insulating material on asemiconductor wafer in which a bottom surface of the wafer iselectrically coupled to a wafer platform comprises exposing the wafer toa plasma including a conductive material. A first layer of theconductive material is formed over the insulating layer until a topsurface of the wafer is electrically coupled to the wafer platform. Thewafer platform is electrically biased to a predetermined potential and asecond layer of the conductive material is then formed over the firstlayer of conductive material.

According to a further aspect of the present invention, a process ofapplying conductive material over a layer of insulating material on asemiconductor wafer comprises positioning the wafer on a wafer platformsuch that a portion of a top surface of the wafer platform is exposedand a bottom surface of the wafer is electrically coupled to the waferplatform. The wafer and the wafer platform are exposed to a plasmaincluding a conductive material to form a first layer of conductivematerial over the layer of insulating material on the wafer surface. Thewafer platform is electrically biased to the wafer platform to apredetermined potential, and a second layer of the conductive materialis then formed over the first layer of conductive material. The processmay further comprise the step of securing the wafer to the waferplatform using a clamping ring engaging a portion of a top surface ofthe layer of insulating material. The process may further comprise thestep of electrically coupling the clamping ring to the wafer platform.

According to a still further aspect of the present invention, a processof forming conductive material on a layer of insulating material on asemiconductor wafer comprises positioning the wafer on a wafer platformsuch that a bottom surface of the wafer is electrically coupled to thewafer platform. The wafer is secured to the wafer platform using aclamping ring engaging a portion of a top surface of the layer ofinsulating material. The clamping ring is then electrically coupled tothe wafer platform. The wafer is exposed to a plasma including theconductive material and a first layer of the conductive material of atleast 100 Angstroms is formed over the layer of insulating material. Thefirst layer contacts the clamping ring such that a top surface of thewafer is electrically coupled to the wafer platform through the clampingring. The wafer platform is then electrically biased to a predeterminedpotential, and a second layer of the conductive material is formed overfirst layer of the conductive material.

A process of fabricating a semiconductor wafer comprises providing awafer having a substrate assembly which has at least one semiconductorlayer. A layer of insulating material is formed over the at least onesemiconductor layer. The wafer is then positioned on a wafer platformsuch that a bottom surface of the wafer is electrically coupled to thewafer platform. The wafer is secured to the wafer platform using aclamping ring engaging a portion of a top surface of the layer ofinsulating material. The clamping ring is electrically coupled to thewafer platform. The wafer is exposed to a plasma including a conductivematerial for forming a first layer of the conductive material over thelayer of insulating material. The first layer contacts the clamping ringsuch that a top surface of the wafer is electrically coupled to thewafer platform through the clamping ring. The wafer platform is thenelectrically biased to a predetermined potential, and a second layer ofthe conductive material is formed over first layer of the conductivematerial.

Accordingly, it is an object of the present invention to provide aprocess of depositing conductive material over a layer of insulatingmaterial in which the risk of arcing through the wafer is reduced. It isa further object of the present invention to provide a process which isinexpensive, easy to implement, does not entail excess processing steps,and does not adversely affect the quality of the deposited conductivelayer. Other features and advantages of the invention will be apparentfrom the following description, the accompanying drawings and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a wafer platform with a wafer positionedthereon according to an embodiment of the present invention;

FIG. 2 is a cross-sectional side view of the wafer platform and wafer ofFIG. 1;

FIG. 3 is a schematic diagram of a bias circuit for the wafer platformof FIG. 1;

FIG. 4 is a cross-sectional side view of the wafer platform and wafer ofFIG. 1 after deposition of a layer of conductive material;

FIG. 5 is a plan view of a wafer platform with a wafer positionedthereon according to another embodiment of the present invention; and

FIG. 6 is a cross-sectional side view of the wafer platform and wafer ofFIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, an embodiment of the present inventionis shown. A semiconductor wafer 10 is shown mounted on a top surface 12Aof a wafer platform 12. The wafer 10 includes a semiconductor layer 16,which is silicon in the illustrated embodiment, and may also includeadditional layers or structures which define active or operable portionsof semiconductor devices (not shown). The wafer 10 may comprise a numberof active semiconductor areas within and over the semiconductor layer 16defining electronic components on one or more individual dies. The wafer10 also comprises an insulating layer 17 formed over the semiconductorlayer 16 and/or active semiconductor areas using conventional methodssuch that a top surface 10A of the wafer 10 is insulated. The waferplatform 12 comprises a conductive material, aluminum in the illustratedembodiment, which serves as a cathode in a plasma deposition chamber(not shown). In the illustrated embodiment, a plasma comprising aconductive material is generated for deposition over the insulatinglayer 17.

The wafer 10 is secured to the wafer platform 12 by a clamping ringmechanism 18 which comprises a clamping ring 20 having a plurality ofclamping fingers 22 thereon. The clamping fingers 22 peripherally engagethe top surface 10A of the wafer 10 and press it against the top surface12A of the wafer platform 12. The clamping ring 20 ensures that a bottomsurface 10B of the wafer 10 substantially engages the top surface 12A ofthe wafer platform 12. The clamping ring mechanism 18 also includes adrive mechanism 24 coupled to the clamping ring 20 for lowering andraising the clamping ring 20 into a clamped and unclamped position,respectively. The clamping ring 20 along with the clamping fingers 22are composed of a suitable conductive material. The clamping ring 20 iselectrically coupled to the wafer platform 12 by a shorting strap 25.Accordingly, the clamping ring 20 and the wafer platform 12 aremaintained at the same potential. It will be appreciated by thoseskilled in the art that the clamping ring mechanism 18 may have avariety of different configurations.

As shown in FIG. 2, the wafer platform 12 includes a heater system 26for heating the wafer 10. The heater system 26 includes a heater 28, awafer seal ring 30, and a gas supply tube 32. The heater 28 is thermallycoupled to the wafer platform 12 and is sized to sufficiently heat thewafer 10. The wafer seal ring 30 provides a seal around the bottomsurface 10B of the wafer 10 to permit heat conductive gas, such asargon, to be circulated over the bottom surface 10B. The heat conductivegas is supplied through the gas supply tube 32 to a plurality ofcylindrical bores 34. Each of the bores 34 connects to an associatedradial groove 36 in the top surface 12A of the wafer platform 12. Thewafer seal ring 30 provides a sufficient seal between the wafer 10 andthe wafer platform 12 so that the wafer 10 is effectively thermallycoupled to the wafer platform 12. The heater system 26 may also includea temperature measuring device (not shown) and a cooling system (notshown) for accurately regulating the temperature of the wafer 10. Itwill be appreciated by those skilled in the art that other systems maybe used to heat the wafer 10 and to establish an effective thermalcouple between the wafer 10 and the wafer platform 12.

The bottom surface 10A of the wafer 10 is also electrically coupled tothe wafer platform 12. As shown schematically in FIG. 3, the waferplatform 12 is electrically biased via a circuit 38. The circuit 38includes a blocking capacitor 40, a matched network 42 and a powersource 44. The power source 44 generates an A.C. or r.f. signal which istransmitted to the blocking capacitor 40 through the matched network 42.A desired D.C. voltage is developed across the blocking capacitor 40which is present on wafer platform 12. As the plasma comprising theconductive material comprises positively charged ions, the desired D.C.is preferably negative so as to attract these ions. The desired D.C.voltage may be changed by adjusting the magnitude of the A.C. or r.f.signal from the power source 44 so as to control the deposition ofconductive material. While the voltage potential on the clamping ring 20is the same as the voltage potential on the wafer platform 12, the topsurface 10A of the wafer acts as a capacitor because of the insulatinglayer 17.

A layer of conductive material 46 may be formed over the insulatinglayer 17 by first ensuring that a D.C. potential is not appliedexternally to the wafer platform 12. With the power source 44 off, theplasma comprising the conductive material is generated. As shown in FIG.4, a first layer of conductive material 46A is formed over theinsulating layer 17. The first layer 46A is grown until enoughconductive material is deposited so as to form an electrical connectionwith the clamping ring 20. Once an electrical connection is formed, thetop surface 10A of the wafer 10 is electrically coupled to the waferplatform 12 through the clamping ring 20 and the shorting strap 25. Withthe top surface 10A shorted to the wafer platform 12, the potentialdifference between the top surface 10A and the bottom surface 10B of thewafer 10 is zero so that arcing through the wafer 10 is eliminated. Thepower source 44 is then energized and a second layer of conductivematerial 46B is formed over the first layer 46A, thereby forming thelayer 46 to the desired thickness. The topology of the layer 46 is thencontrolled by adjusting the magnitude of the A.C. or r.f. signal fromthe power source 44.

The conductive material may comprise any desired conductive material.Metallic materials, and particularly, refractory metals, and theiralloys and compounds may be used to form the layer 46. For example,titanium or titanium nitride, may be used to form the layer 46, astitanium is commonly used to form interconnects between semiconductorcomponents. Further, conductive semiconductor materials, such as dopedpolysilicon, may also be used to form the layer 46.

In the illustrated embodiment, at least 50 Angstroms, and typically,about 100 Angstroms to about 200 Angstroms of conductive material willbe deposited over the insulating layer 17 prior to energizing the powersource 44. It will be appreciated by those skilled in the art that theexact thickness of material which makes the top surface 10A conductiveis dependent, in part, on the type of conductive material. The exactthickness is not important as long as the top surface 10A is conductiveand shorted to the wafer platform 12 through the clamping ring 20 priorto electrically biasing the wafer platform 12. It should be apparentthat the top surface 10A is sufficiently conductive once a substantiallycontinuous layer of conductive material is formed.

Referring now to FIGS. 5 and 6, with like numbers corresponding to likenumbers, another embodiment of the present invention is shown. In thisembodiment, the wafer 10 is not heated but rather subjected to a r.f.bias. Correspondingly, the clamping ring 20 is not required to hold thewafer 10 against the wafer platform 12. The clamping ring 20 is removedfrom the wafer platform, spread outwards from the peripheral edge of thewafer 10 or is merely larger than the wafer 10, thereby exposing aportion 12B of the wafer platform 12 to the plasma. As the portion 12Bof the wafer platform 12 is exposed to the plasma, the wafer platform 12is charged along with the wafer 10, albeit at a slightly different rate.However, the potential difference between top surface 10A and the bottomsurface 10B of the wafer 10 is not large enough to cause arcing througharcing. Consequently, the layer of conductive material 46 may be grownover the insulating layer 17 with the power source 44 energized from thestart. The exposed portion 12B helps reduce the potential differenceacross the wafer 10 such that the risk of arcing is significantlyreduced. It will be appreciated by those skilled in the art that theclamping ring 20 may be used to hold the wafer 10 against the waferplatform 12 so that the wafer is not electrically biased as long as theclamping ring 20 is configured to expose the portion 12B to the plasmaso that the potential difference across the wafer is reduced. The riskof arcing may be further reduced by combining both embodiments of thepresent invention with the portion 12B of the wafer platform 12 exposedto the plasma and a layer of conductive material 46A formed on the topsurface 10A of the wafer prior to energizing the power source 44.

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims.

What is claimed is:
 1. A process of applying conductive material over alayer of insulating material on a semiconductor wafer, wherein a bottomsurface of said wafer is electrically coupled to a wafer platform, saidprocess comprising: forming a first layer of said conductive materialover said insulating layer while said wafer is not electrically biaseduntil said first layer of said conductive material electrically couplesa top surface of said wafer to said wafer platform, thereby providing asubstantially zero potential difference between said top and bottomsurfaces of said wafer to prevent arcing through said wafer; thenelectrically biasing said wafer platform; and then forming a secondlayer of said conductive material over said first layer of conductivematerial.
 2. The process of claim 1 wherein said conductive materialcomprises metallic material.
 3. The process of claim 2 wherein saidmetallic material comprises a refractory metal.
 4. The process of claim1 wherein said conductive material comprises semiconductor material. 5.The process of claim 1 wherein said first layer is deposited to athickness of at least 50 Angstroms prior to electrically biasing saidwafer platform.
 6. The process of claim 1 wherein said first layer isdeposited to a thickness ranging from about 100 Angstroms to about 200Angstroms prior to electrically biasing said wafer platform.
 7. Theprocess of claim 1 wherein said wafer is coupled to said wafer platformby a clamping ring which engages a portion of a top surface of saidwafer and presses said wafer against said wafer platform.
 8. The processof claim 1 wherein said clamping ring is electrically and physicallycoupled to said wafer platform.
 9. The process of claim 8 wherein aportion of a top surface of said wafer platform is covered by said waferand said clamping ring.
 10. A process of applying conductive materialover a layer of insulating material on a semiconductor wafer, wherein abottom surface of said wafer is being electrically coupled to a waferplatform, said process comprising: exposing said wafer to a plasmacomprising said conductive material; then forming a substantiallycontinuous first layer of said conductive material over said insulatinglayer while said wafer is not electrically biased; then electricallybiasing said wafer platform; and then forming a second layer of saidconductive material over said first layer of conductive material. 11.The process of claim 10 wherein the step of exposing said wafer to aplasma comprising said conductive material comprises the step ofexposing a portion of said wafer platform to said plasma.
 12. A processof applying conductive material over a layer of insulating material on asemiconductor wafer, wherein a bottom surface of said wafer iselectrically coupled to a wafer platform, said process comprising:exposing said wafer to a plasma comprising said conductive material;then forming a first layer of said conductive material over saidinsulating layer while said wafer is not electrically biased until a topsurface of said wafer is electrically coupled to said wafer platformthereby providing a substantially zero potential difference between saidtop and bottom surfaces of said wafer to prevent arcing through saidwafer; then electrically biasing said wafer platform; and then forming asecond layer of said conductive material over said first layer ofconductive material.
 13. A process of applying conductive material overa layer of insulating material on a semiconductor wafer, said processcomprising: positioning said wafer having said layer of insulatingmaterial thereon on a wafer platform such that a portion of a topsurface of said wafer platform is exposed and a bottom surface of saidwafer is electrically coupled to said wafer platform; then exposing saidwafer and said wafer platform to a plasma comprising said conductivematerial; then forming a first layer of said conductive material oversaid layer of insulating material while said wafer is not electricallybiased; then electrically biasing said wafer platform; and then forminga second layer of said conductive material over said first layer ofconductive material.
 14. The process of claim 13 further comprising thestep of securing said wafer to said wafer platform using a clampingring, said clamping ring engaging a portion of a top surface of saidlayer of insulating material.
 15. The process of claim 14 furthercomprising the step of electrically coupling said clamping ring to saidwafer platform.
 16. A process of forming conductive material on a layerof insulating material on a semiconductor wafer, said processcomprising: positioning said wafer on a wafer platform such that abottom surface of said wafer is electrically coupled to said waferplatform; securing said wafer to said wafer platform using a clampingring, said clamping ring engaging a portion of a top surface of saidlayer of insulating material; electrically coupling said clamping ringto said wafer platform; exposing said wafer to a plasma comprising saidconductive material; forming a first layer of said conductive materialof at least 100 Angstroms thickness over said layer of insulatingmaterial until said first layer of said conductive material contactssaid clamping ring such that a top surface of said wafer is electricallycoupled to said wafer platform through said clamping ring therebyproviding a substantially zero potential difference between said top andbottom surfaces of said wafer to prevent arcing through said wafer;electrically biasing said wafer platform; and forming a second layer ofsaid conductive material over first layer of said conductive material.17. A process of fabricating a semiconductor wafer comprising: providinga wafer having at least one semiconductor layer; forming a layer ofinsulating material over said at least one semiconductor layer;positioning said wafer on a wafer platform such that a bottom surface ofsaid wafer is electrically coupled to said wafer platform; securing saidwafer to said wafer platform using a clamping ring, said clamping ringengaging a portion of a top surface of said layer of insulatingmaterial; electrically coupling said clamping ring to said waferplatform; exposing said wafer to a plasma of conductive material;forming a first layer of said conductive material over said layer ofinsulating material until said first layer of said conductive materialcontacts said clamping ring such that a top surface of said wafer iselectrically coupled to said wafer platform through said clamping ringthereby providing a substantially zero potential difference between saidtop and bottom surfaces of said wafer to prevent arcing through saidwafer; electrically biasing said wafer platform; and forming a secondlayer of said conductive material over first layer of said conductivematerial.
 18. A process of applying conductive material over a layer ofinsulating material on a semiconductor wafer, wherein a bottom surfaceof said wafer is being electrically coupled to a wafer platform, saidprocess comprising: electrically biasing said wafer platform; exposingsaid wafer and a portion of said wafer platform to a plasma comprisingsaid conductive material; and then forming a substantially continuousfirst layer of said conductive material over said insulating layer. 19.The process of claim 18 further including forming a second layer of saidconductive material over said first layer of conductive material. 20.The process of claim 18 wherein said electrical biasing is a radiofrequency bias.